Bamboo forest was a dominantly ecological and sustainable economic recources. However, bamboo extractives and hemicellulose were nearly complete dissolved out in preparation of the fiber, this would result in more serious water pollution and meanwhile bring a great waste of biological resources. Thereby, In order to obtain the way to lower the negative impact of the extractives and hemicellulose, the Phyllostachys pubescens biomass was treated in alkaline solutions at room temperature and the extractives obtained was analyzed by SIZER basen on the full-factor method. The results were following as: (1) Sort of alkaline and concentration of alkaline had a significant influence on the leaching rate of particles at 0.01 level. And the optimal concentrations of KOH, NaOH, LiOH were 16.0%, 17.5% and 17.5% at 25°С, respectively. (2) Pretreatment, extraction time and sort of alkaline had no influence on the leaching rate of particles. But the benzene/ethanol pretreatment was valuable and necessary. when 16% NaOH and 17.5% KOH were used, the optimal extraction time were all 16h to untested and tested samples.